1. Field of the Invention
The present invention relates to a pattern projection method and a projection system employed for lithography and the like in the production of semiconductor integrated circuits and, more specifically, it relates to a method and a system whereby a mask pattern is projected onto a radiation sensitive substrate by using a charged particle beam such as an electron beam, an ion beam or the like.
2. Description of the Related Art
More and more research into electron beam exposure systems, which achieve both higher resolution and higher throughput in lithography, has been conducted in recent years. As an exposure method employed in this type of system, the method whereby projection is performed for one die (corresponding to one of the many integrated circuits formed on one wafer) or for a plurality of dies, has been examined in the prior art.
However, this method causes a number of problems that must be solved, such as the difficulty in producing a mask to be used as an original template for projection and the difficulty in keeping any aberration in the optical system within allowable limits in a large optical field of one die or more. As a solution, the partial projection method, whereby one or a plurality of dies are projected after being partitioned into smaller areas has been examined recently. In this case, pattern projection is performed by deflecting an electron beam by which specific locations on a mask and a radiation sensitive substrate such as wafer or the like are irradiated while continuously moving the mask and the radiation sensitive substrate. It is to be noted that the deflection of the electron beam is performed based upon the mask position and the radiation sensitive substrate position, which are detected by a detector such as a laser interferometer or the like.
With this method, since the irradiation range per shot of the electron beam is small, it is possible to perform pattern projection at a high resolution and with a high degree of accuracy by reducing the aberration in the optical system.
In the partial projection method described above, a mask provided with punched holes in correspondence to the pattern form, i.e., a so-called stencil mask, has been mainly used. However, as shown in FIG. 12, an island pattern, in which a beam limiting area 1 (the area where the charged particle beam is scattered or absorbed) is surrounded by a punched hole 2 for beam transmission, cannot be provided. In order to project such an island pattern, first, the punched hole 2 in FIG. 12 must be divided into two punched holes 2a and 2b, as shown in FIGS. 13A and 13B, and they must be separately formed on separate masks 3a and 3b respectively. Then, when projecting the patterns 2a and 2b of these masks 3a and 3b onto a radiation sensitive substrate such as a wafer, the projection patterns must be patched together on the radiation sensitive substrate to project the punched hole 2. In this case, the throughput is reduced due to switching of the mask 3a and 3b and the resulting activation and stoppage of the projection system.
In addition, in the projection method described above, a problem arises in that blurring occurs in the projection pattern in the following cases.
In the first such case, the ratio of the projection pattern projected onto a wafer and the pattern on a mask, that is the reduction ratio, and the ratio of the moving speed of the wafer stage and the mask stage, i.e., the drive devices for continuously moving the wafer and the mask respectively, do not match each other.
In the second such case, when irradiating a mask with a charged particle beam, the actual irradiation position deviates from the preset position resulting in the image of the pattern projected onto the wafer deviating from a specific position.
In the third such case, due to a delay generated between the time when the positions of the mask and the wafer are detected and the time when the electron beam is deflected, the image of the pattern projected onto the wafer becomes deviated from a specific position.